This invention relates to a method for regulating DC current. Specifically, it relates to a telecommunication device for regulating the DC line current on a telephone line to conform to desired parameters, and more particularly, to regulate DC line current in accordance with telecommunication requirements of varying countries.
Telephone systems in countries throughout the world have unique system requirements that need to be followed in order to legally sell and use telecommunication devices within their respective borders. One of the commonly known system requirements mandates that when a telephone line goes off-hook (i.e., when the telephone line is in use), the DC current level on the line must reach a certain level within a specified period of time and maintain that level until the call is completed. The DC current level on the line must stay at a certain level in order to be interpreted by the telephone system as an active line throughout the duration of the telephone call. The current rise time and maximum current level are also regulated to prevent damage to telecommunication equipment.
In order to hold a telephone line in the off-hook condition, a specified level of current must be drawn which relates to the voltage level on the line and conforms to a country""s telecommunication requirements. The desired operating current is generally expressed on a graph of current-versus-voltage, known in the art as a load-line. The load-line represents a level of resistance for voltages on a current-versus-voltage graph, allowing a level of current to be determined for a given voltage. FIG. 13 is an example of a current-versus-voltage load-line requirement to keep a telephone line in an off-hook condition. The slope of the load-line on a current-versus-voltage graph is the inverse of the line resistance. Telephone systems develop a voltage which is a potential impressed on the telephone line between two terminals, commonly known as the tip and ring voltage. As seen in FIG. 13, the desired level of current to keep a telephone line in the off-hook condition can be achieved for a given voltage by setting an appropriate line resistance. The template illustrated in FIG. 13 is representative of the parameters set forth by a country and varies from country to country. The parameters can even change within a country due to changes in a country""s requirements (e.g., if a country updates their telecommunication system).
One method that has been used to set the DC line current on a telephone line when the telephone line goes off-hook is to place an inductor in series with a resistor across a telephone line connection and then couple the voice circuits to the line through a capacitor. As shown in FIG. 14, a commonly known prior art circuit for setting DC line current comprises resistance RDC, capacitance C and inductance L. Since inductors appear as shorts to DC current and as high impedance to AC current, the AC current is filtered out leaving just the DC current on the telephone line. The DC current can be set by choosing an appropriate value for R, dependent on the particular DC current level required. The circuit of FIG. 14 is less than optimal because of the inherently bulky nature and high cost of the inductor L, the amount of time for inductor L to charge, and the need to change circuit elements in countries with different off-hook current level requirements.
Another prior art approach that has been used to control the DC line current in a telephone system replaces the inductor L of FIG. 14 with additional system components that are smaller and less expensive. The arrangement of components as shown in FIG. 15 can be used to control DC line current and is commonly known in the industry as a gyrator. The prior art gyrator depicted in FIG. 15 can be used to control DC line current without the use of an inductor L. The circuit in FIG. 15 functions like a large inductor across the telephone line and can be used in place of the prior art circuit shown in FIG. 14. The gyrator is implemented with many discrete components such as transistors, resistors, capacitors, and digitally controlled switches located close to the tip and ring telephone line interface. As shown in FIG. 15, the gyrator contains digitally controlled switches DCSC and DCSR used to switch different levels of capacitance and resistance into the gyrator circuit, respectively. By switching different levels of capacitance and resistance into the circuit, the time constant of the circuit can be changed, such that the transistors can be manipulated to provide the correct level of current on the telephone line within a specified period of time. The circuit allows different start up transient times and DC current levels to be adjusted in accordance with a user""s specifications using a single circuit. The DCSC switches affect initial transient settling time and the DCSR switches affect the DC load-line. However, the adjustability of the circuit is set when the circuit is manufactured, limited by the physical components used in the circuit. If the specifications change after manufacture, in order to change the device, components need to be physically changed within the device or an entirely new device needs to be installed.
An additional requirement that is encountered in the telecommunication industry is that most countries require certification of a device that interfaces with their telecommunication systems. Countries require certification to ensure that a device conforms to their system requirements in order to prevent damage to their telecommunication systems. Researching varying country""s telecommunication requirements is called homologation within the telecommunications industry. Homologation involves going through the telecommunication system requirements of each country where the device is to be used to determine system requirements for licensing. Based on a country""s requirements, determined through homologation, a device may need to be re-certified if the device is changed. Generally, if hardware is changed, countries require that the device be re-certified. On the other hand, if the change is only software related, it is usually only necessary to file a certificate identifying the software change.
The present invention provides a digital method and apparatus for controlling the DC line current on a telephone line that allows control of DC line current settings through the use of a predefined process program. The invention removes the analog gyrator, which contains cumbersome discrete analog components, and replaces the gyrator""s function with a digital gyrator. The digital gyrator controls the DC line current parameters with a predefined process program, implemented with a processor, instead of electrical components such as resistors and capacitors.
The present invention introduces a method for regulating the DC line current on a telephone line with a telecommunication device by interfacing the telecommunication device with a telephone company central office, deriving a first analog signal from the telecommunication device which is representative of the existing DC voltage across the telecommunication device, converting the first analog signal to a first digital signal, filtering the first digital signal to derive a second digital signal, converting the second digital signal to a second analog signal, and regulating a current source with the second analog signal, wherein the current source controls the DC line current on the telephone line.
The present invention further sets forth an apparatus for regulating the DC line current on a telephone line which comprises a data access arrangement coupled with the telephone line, a converter for converting an analog signal produced by the data access arrangement to a digital signal, a processor for filtering the digital signal, and a controllable current source controlled by the processor for setting the DC current on the telephone line.
In accordance with one embodiment of the invention, the processor acts as a digital filter with a first cutoff frequency to allow the DC line current to quickly increase to a satisfactory level when the line is switched to the off-hook position and then switches to a filter with a second cutoff frequency to maintain the DC line current level for the duration of the call. The digital filter is a predefined process program which can be designed to control the characteristics of the filter. By digitally controlling the characteristics of the filter, country specific requirement changes can be accommodated with software, resulting in improvements in cost, distribution, and customer service.